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Development of Xenoturbellida

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Part of the Results and Problems in Cell Differentiation book series (RESULTS, volume 68)

Abstract

Xenoturbellida is a group of benthic marine invertebrates with a very simple bilaterian body plan. Together with Acoelomorpha, it constitutes the clade Xenacoelomorpha, recently interpreted as a sister group to all other extant bilaterians (=Nephrozoa). Therefore, it occupies an important phylogenetic position for studies on origins and evolution of Bilateria. However, due to a number of reasons, developmental and reproductive studies on Xenoturbella have been scarce. In this chapter, I will summarize what is known concerning the reproduction and development of Xenoturbellida, including information from five species that were recently discovered. Gonads are absent in xenoturbellids, and gametes are found in various parts of the body. The gametes are released through ruptures of the body wall, and therefore, the fertilization is presumably external. After holoblastic radial cleavage, embryos hatch as free-swimming hatchlings with uniform ciliation over the surface. Apical tuft forms later in development, and 5 days after hatching, the larvae begin to settle on the substrate. Comparison with the other marine invertebrate larvae suggests that the morphologically simple swimming larvae described in Xenoturbella represent an ancestral larval type of all metazoans and bilaterians.

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© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Shimoda Marine Research CenterUniversity of TsukubaShimodaJapan

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